1. Field of the Invention
The present invention relates to an electric double layer capacitor, and more particularly, to an electric double layer capacitor in which electric charge is accumulated within an electric double layer formed on an interface between an electrolytic solution and a polarized electrode.
2. Description of the Related Art
An electric double layer capacitor has various forms such as a layered type, a cylindrical type, and a button type. However, basically, the electric double layer capacitor is provided with a plurality of single cells (hereinafter, a constructional component having a positive electrode layer and a negative electrode layer, which are formed so as to face with each other through a separator, is referred to as “single cell”) formed of positive electrode layers and negative electrode layers each containing as main components carbon particles such as activated carbon formed on a surface of the electric current collector, and a separator for electrically insulating the positive electrode layers and the negative electrode layers from each other while conducting ions, in which the plurality of cell layers are laminated and received in an exterior case impregnated with an electrolytic solution.
Further, the electric double layer capacitor uses an electrostatic capacity of electric double layers, which are formed on surfaces of insides of micropores of the carbon particles of polarized electrodes (refers to “positive electrode layer and negative electrode layer”) within the electrolytic solution.
The positive electrode refers to a structure in which the positive electrode layer is formed on the surface of the electric current collector, and the negative electrode refers to a structure in which the negative electrode layer is formed on the surface of the electric current collector.
Note that, the electrode layer refers to the positive electrode layer and negative electrode layer, collectively.
The electric double layer capacitor has a feature of having a large energy storage capacity in comparison with general capacitors such as an aluminum electrolytic capacitor, a ceramic capacitor, and a film capacitor.
The electric double layer capacitor also has a feature of having a high power density in comparison with batteries such as a lead battery, a nickel hydrogen battery, and a lithium ion battery.
The electric double layer capacitor is becoming widely used in applications such as an instantaneous voltage drop compensator, a backup for electronic equipment, a power assist for consumer electronic equipment and copying machines, a power supply for a start-up of an automobile after an idle stop, a power supply for a hybrid automobile, and a power buffer for relaxing and leveling variations of photovoltaic power generation and wind power generation. Accordingly, the electric double layer capacitor is expected as an energy storage device, which is useful for spreading and promoting energy conservation and a new energy.
Further, because no chemical reaction occurs in charging and discharging, the electric double layer capacitor has advantages in that a large amount of current is allowed to flow instantaneously, and that charge and discharge efficiency is high. In addition, the electric double layer capacitor has other advantages in that 100,000 times or more of charging and discharging are possible, and that the life-time thereof is ten years or more and the reliability is high.
However, the energy storage capacity of the electric double layer capacitor is low in comparison with a lead battery, a nickel hydrogen battery, a lithium ion battery, or the like. Accordingly, how the energy storage capacity of the electric double layer capacitor can be enhanced is the largest problem to be solved in order to promote the use of the electric double layer capacitor.
Therefore, in order to expand the amount of energy which can be stored in the electric double layer capacitor, there is generally employed a method of expanding an energy storage capacity by: providing positive electrode layers and negative electrode layers having a thickness of about 0.1 mm on both sides of an electric current collector; arranging a plurality of single cells via a separator interposed therebetween and connecting them in parallel; and by increasing a number of the single cells to be received in a single electric double layer capacitor (refer to JP 2003-124078 A, for example).
Further, by increasing the thickness of the electrode layer and using the electrode layer having the thickness of 0.4 mm or 1 mm, for example, the energy storage capacity can be increased. However, the electrostatic capacity and an internal electric resistance become large, resulting in the large current not being allowed to flow instantaneously. In other words, there arises a problem in that the largest advantage of the electric double layer capacitor is eliminated, because the power density thereof decreases.
The larger the electrostatic capacity becomes, the more energy storage capacity can be obtained. However, the necessary discharge time increases, resulting in decreasing of the power density.
Further, as the internal resistance increases, thermal loss that generates during the current flow increases, so it becomes impossible to allow the large current to flow, resulting in decreasing of the power density.
So, in order to suppress the decrease in the power density while keeping the energy storage capacity, there is proposed a circuit structure in which the electric double layer capacitor having a large electrostatic capacity and a large internal resistance and the electric double layer capacitor having a small electrostatic capacity and a small internal resistance are connected in parallel via an external circuit (refer to JP 06-351159 A, for example).
An electrostatic capacity is represented by farad (F) and an internal resistance is represented by ohm (Ω), and the product therebetween is called a normalized internal resistance (ΩF) and corresponds to time constant.
Then, the electric double layer capacitor having the smaller normalized internal resistance is superior in instantaneous power, but is small in retaining force, whereas the electric double layer capacitor having the larger normalized internal resistance is superior in retaining force, but is small in instantaneous power.
In claims and embodiments of JP 06-351159 A, there is disclosed an embodiment of connecting the electric double layer capacitor having a small normalized internal resistance and the electric double layer capacitor having a large normalized internal resistance via an external circuit.
In the internal resistance, ion diffusion resistance which generates at the time of loading and discharging an electrolyte solution to/from pores of the carbon particles is dominant, and the contribution of the electrode thickness and the separator thickness to the internal resistance is relatively small.
Accordingly, in order to increase the power density by lowering the time constant or the normalized internal resistance (ΩF), it is effective to lower the electrostatic capacity.
However, lowering the electrostatic capacity causes lowering of the energy storage capacity, the relation being a dilemma.
Further, as another conventional art, there is disclosed an electric double layer capacitor having a structure, in which a single cell constructed of a thick electrode layer having a large electrostatic capacity and a single cell constructed of a thin electrode layer having a small internal resistance are connected in parallel and received in the same exterior case (refer to JP 08-45793 A, for example).
In the above structure, if the constructional material is the same, the electrostatic capacity can be made larger by thickening the electrode layer. Accordingly, if the internal resistance is the same extent, a single cell having a small normalized internal resistance and a single cell having a large normalized internal resistance may be received in the same exterior case to obtain an electric double layer capacitor.
However, as described in JP 06-351159 A, in the case where an electric double layer capacitor having a large internal resistance and a large electrostatic capacity and an electric double layer capacitor having a small internal resistance and a small electrostatic capacity are connected in parallel via an exterior circuit, a current flows repeatedly between the electric double layer capacitors via electric terminals and wirings connected thereto. Accordingly, current flowing the electric terminals and wirings generates heat to consume much of accumulated energy, thereby causing a problem in that the energy which can be taken out to an outside is markedly reduced.
In addition, as described in JP 08-45793 A, in the case where the electric double layer capacitor in which the single cell constructed of the thick electrode layer having a large electrostatic capacity and the single cell constructed of the thin electrode layer having a small internal resistance are connected in parallel, current flowing the electric terminals, wirings, and the exterior circuits decreases, thereby being capable of preventing the electrostatic capacity, which can be taken out to the exterior from being decreased.
However, there was a serious problem in that the cycle life of the electric double layer capacitor is markedly deteriorated when charging and discharging are repeatedly performed in comparison with the electric double layer capacitor formed of the single cell only, which is constructed of a thick electrode layer having a large electrostatic capacity and with the electric double layer capacitor formed of the single cell only, which is constructed of a thin electrode layer having a small internal resistance.